Fuel cell power systems convert a fuel and an oxidant to electricity. One fuel cell power system type of keen interest employs use of a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of fuels (such as hydrogen) and oxidants (such as air/oxygen) into electricity. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of the stack of fuel cells normally deployed in a fuel cell power system.
As fuel cell power systems are deployed into application having transient power demands such as motor vehicles, fuel cell power system response becomes an issue of concern. In this regard, some components in many fuel cell power systems are designed for operation in a relatively steady state dynamic context where load transients are best accommodated over a relatively long period of time. Vehicles, however, require fairly rapid load change response by the fuel cell power system. In addition to prompt response for the vehicle, the power system ideally maintains nominal voltage output levels during load transients and also effectively handles thermal and/or stoichiometric transients such as carbon monoxide spikes and hydrogen starvation that accompany the load transients.
When a fuel cell power system processes a hydrocarbon by steam reformation and/or partial oxidation to feed high hydrogen content reformate to a fuel cell stack, responsiveness and long-term robustness are needed in both the fuel cell stack and in the reforming process. One problem in this regard occurs when a dramatic upward demand transient on a fuel cell depresses stack output voltage as insufficient hydrogen flows to the fuel cell stack to sustain the voltage during the transient. This condition occurs if the hydrocarbon reforming rate does not accelerate to essentially match acceleration in demand. Another problem is that unacceptably low cell output voltage during the load transient can result from carbon monoxide “spikes” in reformate gas if water vaporization rate change lags the acceleration in load. Reactor durability is also adversely affected as the fuel vaporization rate change lags the acceleration in load and commensurate temperature “spikes” damage the reforming catalyst.
While one solution to the above problems is to delay the response of the vehicle to a change in load command so that essentially steady-state conditions are sustained in the power system, such a solution is unacceptable for drivers conditioned to expect the responsiveness provided by an internal combustion engine. Such a solution is also potentially dangerous for a vehicle operating in a transportation infrastructure built for immediate responsiveness.
What is needed is a fuel cell power system that responds smoothly and comprehensively to load transients. The present invention provides a solution to this need.